The main control indicators for the deep filtration process of granular filter media include: filtration rate and filter cell area, head loss area, head loss and working cycle, the dirt-holding capacity of the filter media layer and the depth of impurity penetration.

(1)Filtration rate and filter area

Filtration rate is the flow rate passing through a unit area, with the unitm/h. Filtration rate is an important indicator for controlling the investment in filter tanks, influencing water quality and operation management. There are two situations of filtration speed: one is the filtration speed under normal working conditions; Another type is the forced filtration rate adopted by the remaining filters to share the processing capacity when a certain filter is unable to operate due to flushing, maintenance or other reasons. The filtration rate is related to the composition of the filter media in the filter tank, as shown in the following table:

Note: The relative density of the filter material is quartz sand2.50~2.70; Anthracite1.4~1.6; Heavy ore 4.40~5.20.

(1)Working cycle and head loss

The period from the start of filtration to the end of flushing in the filter tank is the working cycle of the rapid filter tank. The working cycle is generally12~24hHead loss refers to the difference between the water level on the filter layer during filtration and the water level at the outlet of the main collection pipe after filtration. Head loss is an indicator that determines the flushing of a filter. Generally, the large head loss of a common rapid filter before flushing is approximately controlled at2~3m.

(2)Impurity penetration depth and the dirt-holding capacity of the filter layer

During the filtration process, when a water sample is taken from a certain depth below the surface of the filter layer and precisely meets the filtration water quality requirements, this depth is the penetration depth of impurities. Under the premise of ensuring the quality of the effluent from the filter, the greater the penetration depth of impurities, the better, which indicates that the entire filter layer can fully play its role. Within one filtration cycle, if calculated based on the entire filter layer, the mass of impurities retained in a unit volume of filter material is the dirt-holding capacity of the filter layer, with the unit beingkg/m3. A large dirt-holding capacity indicates that the filter layer plays a significant role in water purification. The dirt-holding capacity of the filter layer is closely related to the depth of impurity penetration.